It has long been known that intermittent drops of power line voltage below certain values caused by power network distribution malfunction can damage various kinds of electric motors and related machinery. Such power line voltage drops can come in two forms. One is the simple power outage wherein the power line voltage drops to zero, and is typically restored in a period of minutes to hours thereafter. This type of power line failure is not particularly damaging, since sensitive electric motors are not forced to operate for a significant period of time at reduced voltage levels, but are simply shut down. The second type consists of outages wherein the power line voltage drops below a safe operating value for periods of several seconds or longer, which may be long enough to damage such equipment.
A well-known solution is to provide for such sensitive installations an undervoltage release unit (tripping unit) which will respond immediately to any power line voltage drop below a given power line voltage (tripping voltage) to actuate an associated circuit breaker to an open circuit condition. The circuit breaker and its associated tripping unit are normally configured so that when the power line voltage drops below the tripping level, the circuit breaker is actuated by the tripping unit to an open-circuit condition and requires subsequent manual resetting by attendant personnel to a closed-circuit condition.
It is during this subsequent reset operation that problems arise. If the operator attempts to reset the circuit breaker before the line voltage is established at a sufficiently high value (reset voltage) the momentary high start-up current of the associated load may reduce the line voltage below the tripping voltage resulting in the circuit breaker opening under high current transient conditions which are very damaging to the circuit breaker contacts.
To obviate such problems, it is known in the art to provide an extension arm or member (release member) operatively coupled to the circuit breaker mechanism and engageable with a solenoid energized from the power line. The solenoid has associated therewith a spring-biased plunger operable between an extended and a retracted position. When the line voltage drops below the tripping level the solenoid releases the plunger, which under the influence of the bias spring, extends to strike the release member to trip the circuit breaker open. Resetting of the circuit breaker is prevented by the spring extended plunger engaging the release member.
In the case of heavy duty circuit breakers this typically requires a relatively massive solenoid coil and a relatively heavy duty plunger spring, leading to space problems and coil heating problems. The solenoid must be capable of retracting plunger at a given reset voltage and thereafter retaining it. Even without such a reset inhibiting feature, powerful solenoids may be necessary for high current circuit breakers that require strong springs to trip the circuit breaker.
There remains a need for a tripping unit (undervoltage release unit) of relatively modest size that is capable of producing a strong tripping force when the line voltage drops below the tripping level, and providing adequate retraction force when the line voltage is once again established at a reset voltage substantially higher than the tripping voltage.